Unfortunately, smoke is usually the first and only sign of the bad sine. In fact, even with a good scope, such oscillations may be too high for it to see.

Of course if the amp has a stable design to begin with...

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- "With power amplifiers, negative feedback is a good thing, and positive feedback can induce destructive oscillation, but with people, positive feedback is a good thing, and negative feedback can induce destructive oscillation"